The design of aircraft to carry radar equipment suitable for carrying out an airborne early warning (AEW) mission poses significant problems. One requirement for such a mission is provision of 360.degree. azimuthal radar coverage. This creates the need to install, in the aircraft, radar antenna arrays having substantial physical space requirements. One solution to this problem is to mount additional structures on a standard aircraft to accommodate the required radar equipment. The well-known addition of an external rotodome onto an aircraft is an exemplary implementation of such a solution. Use of a dorsal fin mounted on an aircraft to house antenna arrays is another known example of such a solution. The addition of such structures typically requires structural modification to the aircraft to accommodate the additional structure.
One obvious adverse affect of such additional structures is that the aircraft suffers aerodynamic drag penalties. As a result, the overall performance of the aircraft is limited. The drag penalties also serve to limit the flying range of the aircraft. This result is contrary to the AEW mission of the aircraft since it is desirable for the aircraft to travel significant distances from its base of operation and/or remain airborne for extended periods in order to scan over a large area. A further adverse effect of additional structures is that the aircraft becomes mission dedicated. That is, the aircraft serves no useful function other than to fly the AEW mission.
A second solution to the problem of accommodating the radar equipment required for an AEW mission is to conformally mount antenna arrays within the aircraft's structure. An example of this solution is disclosed in U.S. Pat. No. 4,336,543 to Ganz et al. In that patent, an array of Yagi end fire antenna elements are conformally mounted along the leading edge of each main wing, along the trailing edge of the tail wing, and in the sides of the aircraft fuselage to provide side-looking antenna coverage. As a result, the aircraft provides 360.degree. of azimuthal coverage. The antenna elements are implemented as a phased array for both transmitting and receiving signals.
With respect to conformally mounted antenna systems such as disclosed in the above noted patent, each antenna element is used both for transmitting and receiving. The instant inventor notes that in view of the substantial amount of transmission power that must be delivered to each antenna element, large conductor electrical cables would have to be routed to the individual antenna elements at all locations. Such heavy cabling represents a substantial weight penalty and further requires allocation of substantial physical space. Further, the use of the individual antenna elements for transmission causes each element to generate significant amounts of heat. This can result in the need for cooling of the antenna elements, e.g., via forced convection. As in the case of the large conductor cabling, provision of cooling equipment represents weight and space allocation problems. Such weight and space problems have a direct impact on aircraft structure and will likely require redesigning of the structure to accommodate the radar antenna arrays and their associated cabling and cooling equipment. The result is an aircraft structural design that is mission dedicated.
With the antenna arrays conformally mounted, either a centrally located single transmitter or a distributed transmitter system can be provided. In the case of the centrally located transmitter, distributor apparatus of a type know in the art is required to divide the transmitted signal and distribute it among the multiple antenna elements. Such distributor apparatus is typically heavy, requires allocation of physical mounting space and therefore adversely impacts the structural design of the aircraft. In a distributed transmitter system, a separate transmitter element is located proximate either each transmitting antenna or group of transmitting antennas. In the case of the conformally mounted antenna array including wing mounted antenna elements, this requires mounting individual transmitters within the wing structure. Further, the individual transmitters typically require connection to a liquid cooling system. As a result, the aircraft structural design must be modified to accommodate the transmitters and their associated cabling and liquid cooling piping. This in turn results in the aircraft structural design becoming mission dedicated. A major disadvantage of a mission dedicated structural design is that the cost of the aircraft design can only be allocated over the limited number of aircraft that are manufactured to perform the particular mission. This results in a substantial cost to be recovered from the sale of each aircraft.